What is DEF?

Diesel Exhaust Fluid (DEF) is an emissions control liquid required by modern diesel engines. It is injected into the exhaust stream. DEF is never added to diesel fuel. It is a non-hazardous solution of 32.5% urea in 67.5% de-ionized water. DEF is clear and colorless, and looks exactly like water. It has a slight smell of ammonia, similar to some home cleaning agents. DEF is used in by Selective Catalytic Reduction (SCR) technology to remove harmful NOx emissions from diesel engines.

In January 2010, the U.S. Environmental Protection Agency (EPA) brought in new emissions standards requiring medium- and heavy-duty vehicles to significantly reduce engine emissions, particularly NOx and particulate matter (PM). Vehicle manufacturers use SCR to meet these standards. DEF is sprayed into the exhaust, breaking down NOx gases into nitrogen and water using an advanced catalyst system. As a result most new diesel trucks, pickups, SUVs, and vans are now fitted with SCR technology and have a DEF tank that must be regularly refilled.

EPA set the emissions standards to improve air quality. NOx and PM emissions are associated with a wide range of health problems including respiratory and cardiovascular diseases, aggravation of asthma, acute respiratory symptoms, chronic bronchitis and decreased lung function. The EPA estimates that the emission standards will prevent 8,300 premature deaths, more than 9,500 hospitalizations and 1.5 million work days lost due to illness, saving approximately $70.3 billion by 2030.

SCR is a so-called "aftertreatment" technology, which means that it destroys harmful emissions after combustion. This gives manufacturers greater scope to tune engines to improve fuel efficiency and increase power. Owners of SCR vehicles enjoy greater reliability and longer oil change intervals, which add up to impressive operating cost savings over the life of the vehicle.